1. Field of the Invention
The present invention relates to a printer with sheet preheating in which a thin film heat transfer toner ribbon material is heat-transferred onto a recording sheet by a thermal head or a laser.
2. Description of the Related Art
Conventionally, for example, as shown in FIG. 8, a printer 1 using a line thermal head includes as main structural members: a thermal head 3; a platen roller 5 which opposes a resistance heat generating body 3a of the thermal head 3; a toner ribbon 7 which is disposed between the thermal head 3 and the platen roller 5 and which is supplied from a conveying side 7a and is taken up onto a winding side 7b; a guide roller 9 which guides the toner ribbon 7; a pinch roller 11 and a capstan roller 13 which are provided at the downstream side of the thermal head 3 in the feeding direction of a recording sheet; and a back-up roller 15 which abuts the pinch roller 11.
As shown in FIGS. 9A through 9C, in order to effect heat recording onto a recording sheet in the printer 1 structured as described above, firstly, a heat adhesive recording sheet (a recording sheet) 17 which has a heat adhesive layer 17a on the surface is fed (see FIG. 9A). The toner ribbon 7 is superposed on the recording sheet 17, and the toner ribbon 7 and the recording sheet 17 are inserted between the thermal head 3 and the platen roller 5. Next, when a portion of the resistance heat generating body dots corresponding to an image to be printed is selectively heated, adhesive strength of the surface of the recording sheet is increased and toner adheres onto the recording sheet 17 from the toner ribbon 7 (see FIG. 9B). Subsequently, when the toner ribbon 7 separates from the recording sheet 17 while the recording sheet 17 is conveyed, only the toner portion t adhered to the surface of the recording sheet is remained and fixed onto the recording sheet 17 (see FIG. 9C). In this way, the image information is recorded onto an image receiving surface of the recording sheet 17.
However, in the aforementioned conventional heat transfer type printer 1, a predetermined amount of thermal energy is supplied to the toner ribbon 7 from the thermal head or laser, and the toner and the surface of the recording sheet are heated for transfer. Accordingly, the heat adhesiveness of the surface of the recording sheet is changed by environment to be used (in particular, temperature) and sensitivity (the degree of amount of transfer relative to the amount of heat to be supplied) varies. Thus, there is a drawback in that recording densities become different.
Namely, in the recording sheet 17 shown in FIG. 10A, a surface polymer which forms the heat adhesive layer 17a tends to harden at low temperature. As a result, the adhesive strength of the recording sheet 17 lowers in low-temperature environment. On the other hand, if the heat adhesive layer 17a is heated in advance, the adhesive strength of the recording sheet remains high even if the recording takes place several hours later.
Therefore, as shown in FIG. 10B, when a plurality of colors are superposed and recorded onto the recording sheet 17, a first color 19a is directly recorded onto the surface of the recording sheet 17 and a second color 19b is superposed and recorded onto the toner of a first color 19 which has been recorded onto the recording sheet 17. In this case, the head adhesive strength of the second color 19b, which is heated again, is higher than that of the first color 19a. As a result, the sensitivities become different and a drawback arises in that the recording densities become different.